1. Field of the Invention
This invention relates to a DC power device including a switching regulator and a linear regulator for applying a DC voltage produced by each regulator to an external load.
2. Discussion of Related Art
In electrical equipment having electrical elements requiring different operating voltages such as, for example, a word processor having a printing motor as an element of a high-voltage driving system, and a document-preparation control circuit as an element of a low-voltage driving system, a DC power device is provided which is capable of individually supplying each electrical element with a voltage suitable as the operating voltage for that electrical element.
A DC power device which supplies different operating voltages is known, and includes a chopper or inverter type of switching regulator and a series or shunt type of linear regulator as shown in FIG. 8. In this DC power device, a DC voltage (VPDC) which is obtained by rectifying and smoothing a commercial voltage is first supplied to the switching regulator 81, which then generates a predetermined high voltage (VPH), suitable for powering electrical elements of the high-voltage system. The high voltage (VPH) generated by the switching regulator is also supplied to the linear regulator 82 to generate a predetermined low voltage (VPL), suitable for powering electrical elements of the low-voltage system.
The conventional DC power device described above can stably generate two types of DC voltages: a high voltage (VPH); and a low voltage (VPL) using the regulators 81 and 82, respectively, and can also stably supply a source voltage (operating voltage) to a load irrespective of fluctuation of the load. However, the linear regulator of this conventional DC power device suffers a large power loss because a linear regulator 82 having a low conversion efficiency is used to convert the high voltage (VPH) to the low voltage (VPL). Additionally, large circuit elements for the regulator, and a large heat sink for dissipating heat generated in the circuit elements are required. Therefore, these DC power devices cannot be used when high efficiency and miniaturization of the DC power device are required.
In order to overcome the above disadvantages, it is proposed to use a switching regulator having high conversion efficiency for the generation of a low voltage as well as a high voltage. However, this proposal has the following disadvantage: in the switching regulator, an input voltage (DC voltage) is once converted into an AC voltage, and then the AC voltage is rectified and smoothed to generate a predetermined DC voltage; thus, when a linear regulator is replaced by the switching regulator, there would occur new problems in that the number of electrical elements is increased, thus increasing the cost of the device, and countermeasures for handling noise must be provided because noise is liable to occur due to a switching operation performed therein.